Fender PRRI might need new tubes

Discussion in 'Amp Tech Center' started by Snowwizard, Jul 31, 2014.

  1. Snowwizard

    Snowwizard Tele-Holic

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    So. Usually I just pay a guy to do this kind of stuff but I'm wanting to get better at DIY. So even though I've been playing a long time, I"m a rookie on this front.

    My Fender '65 Princeton Reverb RI has started to crackle when it's turned up or there's any gain. I'm assuming this is a tube issue.

    My questions are:

    -Is that a reasonable assumption?

    -What is rebiasing?

    -If I just stick the same tubes in, does rebiasing even affect me?


    Thanks.
     
  2. Wally

    Wally Telefied Ad Free Member

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    1) Hmmmm...that assumption could be correct or incorrect. Reasonable....I don't look for reasonable anymore! LOL

    2) Rebiasing is setting the bias voltage so that the plate dissipation does not cause redplating while that current draw yields a sonic that the player desires....there is a range of 'correct' levels of plate dissipation.

    3) Once an amp has been biased correctly and to the player's desires---if the player has gotten to the point of understanding what they want---, then replacing power tubes with tubes with the same performance specs will allow one to replace power tubes without rebiasing and still maintain the same sonics. Tubes from the same manufacturer and of the same model do not necessarily perform the same way in circuit...so, no...just because you buy the same tube doesn't guarantee anything. You have to have tubes that match the tubes that were in the amp when the amp was biased.
    IF you are caught needing to install new power tubes and you can't rebias different tubes at that moment in tiem, then install the tubes, fire the amp...and be watching the power tubes' plates. If the tube's plates start glowing red, then shut the amp down. IF the plates...the long and usually darker elements that run the length of the construction in these tubes....don't turn orange/red, then play while watching. IF still there is no redplating, you can safely play the amp. The sonics may not be the same...but it is safe to play. In fact, the amp may sound terrible because the biasing is too cold. You never know...unless you are using tubes with similar performance specs.
     
  3. syrynx

    syrynx Tele-Afflicted

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    Snowwizard, tubes are easy to blame, because they do fail, and because they're easy to replace. But IME good tubes often take the blame for other issues, including (but not limited to) poor connections in the tube sockets.

    Try this: With the amp cold, remove each tube, then reinsert it into its socket. Do this several times with each tube. The object of this exercise is to try to scrape off anything that may be preventing a proper metal-to-metal connection between the tube pins and the connectors in the sockets. If this changes the amp's misbehavior in any way, you'll know that you have a connection issue. And it costs you only a few minutes.

    Here's a copy 'n' paste of a text file I post here several times a month:

    Your hitherto faithful amplifier has betrayed you. It's working, but not well. Maybe it's cutting in and out. Maybe the volume has dropped significantly, or drops and then comes back to normal intermittently. Maybe it's making noise-- perhaps an irregular, static-like noise, perhaps a continuous white or pink noise, perhaps a sound that doesn't originate from your guitar and only shows up on certain notes. Maybe it's distorting, and not in a good way. Maybe it's a mixture of two or more of these unpleasant malfunctions.

    The first step I would take, because it's the quickest, the easiest, the least expensive, and the most likely to address the problem, is to clean and re-tension all spring tension connections in the amp.

    Even in a very simple amp-- a SF Champ-- there are 31 (yes, thirty-one) spring tension connections. The more complex the amp, the more spring tension connections there are. Some are extremely unlikely to cause either the noise or the intermittency (the power switch, the fuse holder, the speaker jack).

    But many of the others-- the connections in the input jacks, the pot wipers, and the connections between the tube pins and the sockets-- are HIGHLY likely to be point-contact semiconductor diodes at this stage in the amplifier's history.

    Metals oxidize over time, in the presence of, er, oxygen. Oxidation is speeded up by heat, which amplifiers generate. Put a metal point in contact with a thin film of metal oxide, and you get a diode. This ain't new, and it ain't high tech; such diodes were in use long before there were electron tubes.

    Diodes are really good at two things: Rectifying (making AC into pulsating DC) and generating noises: Burst noise, Flicker noise, and Shot noise.

    What you thought was a pure tube amp is almost certain to have a whole flock o' semiconductor diodes you didn't know were there. Make 'em go away, and the noise and intermittents will probably go away as well. Make the metal oxides go away, and the diodes go away. Caig's aptly-named DeoxIT is highly regarded by many in these forums, and I've used it with very good success.

    Our TDPRI brother Billm has a good tutorial on cleaning and re-tensioning tube sockets on his Web site. Our other forum brother andyfromdenver covers servicing input jacks, cleaning pots, and re-tensioning tube sockets in his Service Diary: Silver Face Fender Champ Fun!. Really, you should read the whole thread.
     
  4. muchxs

    muchxs Doctor of Teleocity

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    Dude! It's a reissue. It's a sad commentary on the human condition if reissues corrode that fast especially in the OP's location... dry sunny Napa, California.

    Yes it's a reasonable assumption. It's a reasonable assumption because original Princeton Reverbs had supply voltages higher than anything listed on any 6V6 tube data sheet or example circuit. The supply voltage in the PRRI is approximately 20 volts higher than found in originals and it was too high to begin with!

    The "reissue" tubes are nowhere near as good as the originals.

    Use tubes with a 1000 hour expected lifespan when run the way they were designed to run... you can expect at least a 50% and more likely an 80% reduction in tube life at the elevated supply voltage found in the PRRI.
     
  5. refin

    refin Friend of Leo's

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    I am humbly following muchxs in an amp thread.....:lol:

    I played through a PRRI the other day at the store where I teach and do guitar repairs.Wonderful clean tones,made me wanna play.Yet cranked to 6 or 7,it was very harsh.I never knew they were biased higher! I have a DRRI that was biased just a little north of a freezer.
     
  6. cousinpaul

    cousinpaul Friend of Leo's

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    I'd have a tech look at it before buying tubes. Those PCB Fenders are prone to other issues as well as tube failure. Might be a solder joint that needs to be re-flowed or something.
     
  7. Snowwizard

    Snowwizard Tele-Holic

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    4 to 5 seems to be the sweet spot for the PRRI. I'm fine with it because I need to keep good tone at lower volumes. My take is that if you need it louder then you want a different amp
     
  8. muchxs

    muchxs Doctor of Teleocity

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    It's just plain dumb the fun is over at "5" on a Princeton Reverb. That's like havin' a car that develops a shimmy at 55 mph.

    I once drove a couple hundred miles in a '64 VW that had a shimmy that came on at 35 mph. I figured a couple hundred pounds of rocks packed in the spare tire well would help damp the vibration. Sure enough, it started to shimmy at 45 mph with the spare tire in the back seat and the nose loaded with stone.

    ...almost a 30% improvement! :D


    Dave Allen calls his Princeton Reverb influenced design the "Sweet Spot", fittingly enough. What he's done is expanded the Princeton Reverb's sweet spot so it extends almost all the way up the dial.
     
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